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Planet Mercury
Mercury is the innermost and smallest planet in the Solar System,[a] orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century, a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity.[11] Mercury is bright when viewed from Earth, ranging from −2.3 to 5.7 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. Since Mercury is normally lost in the glare of the Sun, unless there is a solar eclipse it can be viewed from Earth's Northern Hemisphere only in morning or eveningtwilight, while its extreme elongations occur in declinations south of the celestial equator, such that it can be seen at favorable apparitions from moderate latitudes in the Southern Hemisphere in a fully dark sky. Comparatively little is known about Mercury; ground-based telescopes reveal only an illuminated crescent with limited detail. The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which attained orbit around Mercury on March 17, 2011,[12] to map the rest of the planet.[13] Mercury is similar in appearance to the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. Unlike the Moon, it has a largeiron core, which generates a magnetic field about 1% as strong as that of the Earth.[14] It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to 700 K (−183 °C to 427 °C),[15] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest. Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they calledHermes.[16] The English name for the planet comes from the Romans, who named it after theRoman god Mercury, which they equated with the Greek Hermes (Ἑρμῆς). The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.[17]. orbit Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46 to 70 million kilometers. It takes 88 days to complete an orbit. The diagram on the right illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having the same semi-major axis. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet’s rotation around its axis, result in complex variations of the surface temperature.18 This resonance makes a single day on Mercury last exactly two Mercury years, or about 176 Earth days.73 Mercury’s orbit is inclined by 7 degrees to the plane of Earth’s orbit (the ecliptic), as shown in the diagram on the right. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between the Earth and the Sun. This occurs about every seven years on average.74 Orbit of Mercury as seen from the ascending node (bottom) and from 10° above (top) Mercury’s axial tilt is almost zero,75 with the best measured value as low as 0.027 degrees.7 This is significantly smaller than that of Jupiter, which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury’s poles, the center of the Sun never rises more than 2.1 arcminutes above the horizon.7 At certain points on Mercury’s surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four days before perihelion, Mercury’s angular orbital velocity exactly equals its angular rotational velocity so that the Sun’s apparent motion ceases; at perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.18 Spin–orbit resonance After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated. For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and always keeping the same face directed towards the Sun, in the same way that the same side of the Moon always faces the Earth. Radar observations in 1965 proved that the planet has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury’s orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury’s sky.76 The original reason astronomers thought it was synchronously locked was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury’s 3:2 spin–orbit resonance, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days.18 A sidereal day (the period of rotation) lasts about 58.7 Earth days.18 Simulations indicate that the orbital eccentricity of Mercury varies chaotically from nearly zero (circular) to more than 0.45 over millions of years due to perturbations from the other planets.1877 This is thought to explain Mercury’s 3:2 spin-orbit resonance (rather than the more usual 1:1), since this state is more likely to arise during a period of high eccentricity.78 Numerical simulations show that a resonant orbital interaction with Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the planet may collide with Venus within the next five billion years.79 Advance of perihelion Main article: Perihelion precession of Mercury In 1859, the French mathematician and astronomer Urbain Le Verrier reported that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead a series of smaller 'corpuscules') might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation.80 (Other explanations considered included a slight oblateness of the Sun.) The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place faith in this possible explanation, and the hypothetical planet was named Vulcan, but no such planet was ever found.81 The perihelion precession of Mercury is 5600 arc seconds per century. Newtonian mechanics, taking into account all the effects from the other planets, predicts a precession of 5557 seconds of arc per century.82 In the early 20th century, Albert Einstein’s General Theory of Relativity provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds per century, therefore it requires a little over twelve million orbits for a full excess turn. Similar, but much smaller, effects operate for other planets: 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.8384